In the field of quantum cryptography, quantum imaging and quantum computing there is a need to produce pairs of photons. Such photons can be created from a cascade emission process in single quantum dots initially filled with two electrons and two holes, a “biexciton state”. This state can emit “a biexciton photon” leaving one electron and one hole in a “(charge-neutral) exciton” state. This electron and hole then recombine to emit an “exciton” photon leaving the dot empty. Through control of the properties of the exciton state these two photons can be entangled.
When a quantum dot contains only one electron and one hole the spins of these particles are arranged in such as manner that that ensures photons emitted from their decay can have one of two orthogonal linear polarisations, with an energy separation known as the fine-structure splitting. Alternatively, the carriers can be arranged in a superposition of these states, which can the thought of as a qubit. Quantum dots with small fine-structure splitting can be used to generate entangled photon pairs, or to store a qubit in this configuration. In the case where the fine-structure splitting is zero this superposition, once prepared, will persist until the carriers recombine and a photon is emitted. However, if the fine-structure splitting is finite the superposition evolves with time.